Feat: Upsert data when an unique field is provided

[ocarlosapenas]

Proposed changes

Create a feature to give developers the option to activate upsert during the import process. Upsert is set to false by default, but when set to true, it can update records that already exist in the database.

Types of changes

What types of changes does your code introduce to Filament Import?
Put an x in the boxes that apply

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

[adnoh]

@frankyso how do we get this gem finally merged? it works like a charm in the main branch tested locally as well